There really is no tutorial to make a texture, you just... do it. The results depend on how skilled you are with the image software you are using. If you are using PhotoShop, looking for tutorials on how filters work is an option, although experimenting on your own is a better idea.
Things to have in mind. Textures that were 128x128 in HL1 will now be 512x512 to look good.
Start here since this site contains a step-by-step guide.
http://www.hl2world.com/wiki/index.php/Category:Texturing